The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) standards Pub. 950, etc. That is, these standards provide that at least three insulating layers be formed between primary and secondary windings, in which an enamel film which covers a conductor of a winding be not authorized as an insulating layer, or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
According to such the standards, as a currently prevailing transformer has a construction such as the one illustrated in a cross-section of FIG. 2. Referring to FIG. 2, a flanged bobbin 2 is fitted on a ferrite core 1, and an enameled primary winding 4 is wound around the bobbin 2 in a manner such that insulating barriers 3 for securing the creeping distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creeping distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
Recently, a transformer having a construction which includes neither the insulating barriers 3 nor the insulating tape 5, as shown in FIG. 1, has started to be used in place of the transformer having the construction shown in the cross-section of FIG. 2. The transformer shown in FIG. 1 has an advantage over the one having the construction shown in FIG. 2 in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6d), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of primary and secondary windings 4 and 6 used.
As such a winding, a winding in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another, is known. Further, a winding in which a conductor enameled with polyurethane is successively extrusion-coated with fluoroplastics, whereby extrusion-coating layers composed of three layers structure are formed for use as insulating layers, is known (Japanese Utility Model application (JU-A) No. 3-56112).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the wire becomes high. Further, in this case of the insulating layer, there is a problem that, since the insulating layer cannot be removed by dipping in a solder bath, the insulating layer on the terminal has to be removed using less reliable mechanical means, and further the wire must be soldered or solderless-connected, when the terminal is worked for the insulated wire to be connected, for example, to a terminal.
On the other hand, a multilayer insulated wire is put to practical use, wherein multilayer extrusion-insulating layers are formed from a mixture of a polyethylene terephthalate as a base resin with an ionomer prepared by converting part of carboxyl groups of an ethylene/methacrylic acid copolymer to metal salts, and wherein the uppermost covering layer of the insulating layers is made of a nylon. This multilayer insulated wire is excellent in cost of electrical wire (nonexpensive materials and high producibility), solderability (to make possible direct connection between an insulated wire and a terminal), and coilability (that means that, in winding the insulated wire around a bobbin, the insulating layer is not broken to damage the electrical properties of the coil, when, for example, parts of the insulated wire are rubbed with each other or the insulated wire is rubbed with a guide nozzle)(U.S. Pat. No. 5,606,152, and JP-A-6-223634 ("JP-A" means unexamined published Japanese patent application)).
Further, to improve heat resistance, the inventors proposed an insulated wire whose base resin is changed from the above polyethylene terephthalate to polycyclohexanedimethylene terephthalate (PCT).
However, although the heat resistance of these multilayer insulated wires is acceptable to heat-resistance Class E in the test method in conformity to Annex U (Insulated wires) of Item 2.9.4.4 and Annex C (Transformers) of Item 1.5.3 of the IEC 950-standards, it nonetheless cannot meet the increased level of heat resistance demanded in recent years, and it is not acceptable to heat-resistance Class B of the IEC standards.